The topic of the present study is the establishment of a highly sensitive and accurate method to detect the structural modification of DNA caused by the binding of carcinogens. Electron microscopy, gel electrophoresis, high pressure liquid chromatography, and nuclear magnetic resonance (NMR) of carcinogen modified DNA and its hydrolyzed forms allowed us to analyze the interaction between nucleic acids and carcinogens at the molecular level. The effect of carcinogen binding and modification on the genetic function of DNA and particularly on its replication and repair was studied using an in vitro assay system previously established in this laboratory. Modification of DNA at a preselected region by enzymatic and chemical manipulation is also in progress. This system has allowed us to analyze in vivo the effect of carcinogens on the genetic machinery of the cell.